Among the most critical phases during a flight are the landing and the take-off. During landing, it is of critical importance that the position of the aircraft and the sink rate are matched to one another such that the aircraft touches down at a predefined position on the runway, referred to as the nominal touch-down point, such that the pilot has sufficient freedom to allow the aircraft to wheel out and to brake it down to the taxiing speed. Discrepancies of several meters to the left or to the right and excessively early contact with the ground lead to the aircraft leaving the runway and having an accident. In contrast, if the aircraft touches down too late, then there is a risk that it will no longer be possible to brake the aircraft down to the taxiing speed and that the aircraft will roll on the runway, likewise meeting with an accident.
While the pilot can land the aircraft safely in visual flight in good weather and visibility conditions, additional systems, which assist the pilot during the landing, are required in particular in poor weather conditions, for example in fog or in the dark. In this case, systems such as these are used to indicate a path to the touch-down point on the runway threshold to the pilot, which he cannot estimate himself because of the external weather conditions.
By way of example, the ILS (instrument landing system) has been known since the 1920s, with this being a ground-based landing system and assisting the pilot during the landing process, in particular in poor weather conditions. In this case, two electromagnetic guide beams are used to indicate the course which the aircraft must follow in order to touch down safely on the runway. A corresponding receiver is located on board the aircraft, which receives the electromagnetic signal and indicates the appropriate course for the pilot visibly on a display. A further landing system, which was developed in the 1980s, is the MLS (microwave landing system). The system transmits a beam, which is deflected horizontally and vertically in time, into the approach area of the aircraft.
In addition to the two landing systems mentioned above, satellite navigation systems are also used in addition in good visibility conditions nowadays, in order to allow the location position of the aircraft to be determined in three dimensions, with this beam also being included in the landing process. Satellite navigation systems such as GPS (Global Position System), GALILEO, or GLONASS are, however, in general accurate only to a few meters (one to three meters). This error component can admittedly be corrected to an extent with the aid of DGPS (Differential GPS), by transmitting an appropriate correction signal based on the signal measured by a nearby DGPS ground station. However, even this does not achieve the accuracy which is required to allow an aircraft to touch down precisely on the runway threshold. This is because an accuracy of a few centimeters is required for this purpose, since, otherwise, it is not possible to determine the height of the aircraft above ground sufficiently accurately. A further disadvantage is the excessively low clock rate with which the signals are refreshed and which in consequence cannot provide continuous information.
Furthermore, the use of satellite navigation systems is subject to the disadvantage that the legal position of the operator is problematic. In the event of a failure or a fault, the legal question therefore remains as to who is responsible for the resultant damage. In addition, at the moment, satellite navigation systems are not licensed for landing assistance in poor visibility (ICAO-CATIII).
For example, U.S. Pat. No. 6,469,654 B1 discloses a transponder landing system, in which a transmitting unit which is arranged on the ground transmits a signal to the aircraft. This signal is then identified by a transponder, as is required for secondary radar, and is transmitted back, with the signal which is transmitted back being received by a plurality of receivers which are arranged on the ground. The range is then determined on the basis of the signal delay time, and the position of the aircraft can be deduced in this way. U.S. Pat. No. 5,017,930 also discloses a similar system.
However, the considerable disadvantage of both systems is that the processing speed of the transponder is not known, as a result of which this is still unknown when determining the signal delay time which, in the end, leads to increased inaccuracy in the determination of the range.